A heater for steering wheel of an automobile (a heater which is used for steering wheel for changing the direction of movement of the automobile) is spreading as the comfortable equipment of cold districts. In many steering wheels having this heater, the type that the surface skin is covered by leather is mainstream. Concretely, as shown in FIG. 5, the steering wheel consists of a ring portion 61 which is formed in the shape of a ring, a boss portion 62 which is arranged in the almost center position of the ring portion 61, a spoke portion 63 which combines the ring portion 61 and the boss portion 62 by lengthening from the inner diameter side of the ring portion 61 toward the boss portion 62, a planar heat generating element 50 which covers the circumference of the ring portion 61, and a surface skin portion 64 which covers by leather the circumference of the ring portion 61 which is covered by the planar heat generating element 50. And the whole circumference of the steering wheel can be warmed. In addition, the reason why the planar heat generating element 50 for covering the circumference of the ring portion 61 of a steering wheel 60 is used is for not losing the shape when adhering tightly to the ring portion 61 together with the elasticity and flexibility.
As such planar heat generating element 50, for example, the reticulate heater shown in FIG. 6 is disclosed (for example, refer to patent document No. 1). This reticulate beater 50 consists of a reticulate heat generating element 52 that the plurality of heater wire 51 are braided in a mesh form, and an electrode 53 for power feeding is provided at both end portions 52a, 52b of one direction of this reticulate heat generating element 52 in the separated state. The reticulate heat generating element 52 is formed by tricot knitting technique which knits loops by a plurality of heater wire 51 with continuous and planar state in the longitudinal direction. In the heater wire 51, the varnish for enamel wire is coated to the heater bare wire and baked, thereby the insulating sheath is formed, and the electrode 53 is protected by such as insulating tape.
The steering wheel 60 having this reticulate heater 50 has no problem to the insulating property or the waterproofness at the normal use condition. However, when such as sweat or drinking water soaks into the surface skin portion 64 repeatedly and in large quantity, the leakage current occurs by such as sweat or drinking water, and minute defects such as pinholes which occur at the insulating sheath of the heater wire 51 are generated, or the breaking of wire from corrosion of electrolysis by defect of insulating portion of soldered portion of the electrode 53 is generated. That is, as shown in FIG. 7, when applying electric current to the reticulate heater 50 in the water solution, the leakage current concentrates to the exposed portion of a conductor such as minute defects of pinholes and so on by potential difference of the reticulate heater 50 in the water solution, and thereby the electrolysis is caused. For example, these unfavorable cases might occur when the driver whose hands have much amount of sweating grips almost same portion of the steering wheel 60 for the long time, or when the driver slops the drinking water and so on accidentally to the steering wheel 60, or when driving during the long time in the rainy condition on the open car which opens the roof.
For these unfavorable points of the reticulate heater 50, the application of the planar heat generating element which covers the metallic thin film which generates heat by applying electric current by the polyester film with adhesive comprising polypropylene or polyethylene is considered (for example, refer to patent document No. 2). In this planar heat generating element, the hot press machine or the laminating machine is used when bonding the polyester film to the metallic thin film.    Patent document No. 1: Japanese Patent Publication No. 2001-110555    Patent document No. 2: Japanese Patent Publication No. 2002-117961